3GPP, which is a group working on standardization of the third generation mobile communication system, has been conducting studies collectively called “LTE (Long Term Evolution)” in order to achieve significant increase in transmission speed and reduction in transmission delay in a radio access network (RAN: Radio Access Network), and has been pursuing formulation of standardized specifications for constituent technologies relating to the studies.
Additionally, a radio access scheme based on the LTE or the like is configured to employ a DRX (Discontinuous Reception) technology in order to save power consumption in a mobile station UE.
Specifically, in a mobile communication system based on the LTE scheme, a radio base station eNB and a mobile station UE are both configured to change reception cycles of downlink data in the mobile station UE by use of Inactive timers.    Non-patent Document 1: 3GPP TSG RAN WG2 Meeting #57bis R2-071553 (Mar. 26, 2007)